The rat's whiskers are the first step in a highly sensitive sense
of touch. Whiskers brush over the ground, obstacles, food, and other
rats. When a whisker touches an object it bends, and when it bends
its follicle sends a message to the rat's brain. The rats can tell
the direction and how far each whisker moves. As the rat moves
through the landscape he feels all the objects and textures near his
face, and constructs an image of the world around him. This image may
be far more detailed than what the rat can see through his eyes.

But that's not all whiskers do. Recent research has shown that the
rat's whiskers may also be used in hearing.

Schematic drawing of whisker frequency
map. Long whiskers that vibrate at low frequencies are
located at the back of the array; short whiskers that
vibrate at high frequencies are located at the front.
Adapted from Andermann et al. 2004.

Whiskers resonate at certain frequencies, like the strings of a harp
(Neimark et al. 2003, Hartman et al. 2003). Longer
whiskers vibrate at lower frequencies; shorter whiskers vibrate at
higher frequencies. Because the rat has short whiskers near the nose
and long ones further back, his array of whiskers creates an orderly
map of frequencies on his face (Neimark et al. 2003).

Whiskers are organized in a grid with
five horizontal rows and 5 to 9 columns (Vincent
1913)

Each whisker is wired to a specific location in the brain's sensory
cortex (called the barrel cortex). These brain locations are
organized in a map that matches the layout of the whiskers on the
face, and thus matches the resontant frequencies of the whiskers.
Therefore, areas of the brain sensitive to low frequency whisker
vibrations are grouped together, as are areas sensitive to high
frequencies (Andermann et al. 2004). This layout parallels the
neural representation of the auditory system.

Whisker vibration may explain the extreme sensitivity of the rat's
sense of whisker-touch. Rats can make fine distinctions between
different textures, like different grades of sandpaper. Rats may do
this by sweeping their whiskers over the sandpaper and feeling, not
just the whiskers' overall displacement, but also the vibrations of
different whiskers. Coarse textures vibrate the longer whiskers,
while fine textures vibrate the shorter ones.

The curled whiskers of rex rats probably don't resonate as well as
straight whiskers. Therefore, rex rats probably do not 'hear' or
distinguish different textures with their whiskers as well as normal
rats.